1. Field of the Invention
The invention relates to an illumination device for supplying a beam of linearly polarized light having a non-rotationally symmetrical beam cross-section at the location of an object to be illuminated, said device comprising a light source, a reflector, an optical integrator for forming said beam cross-section and for realising a uniform light distribution across the beam cross-section, and a polarizing system for converting the source light into linearly polarized light.
The invention also relates to an image projection apparatus comprising such an illumination device.
2. Discussion of the Related Art
The term image projection apparatus should be considered to have a wide meaning and comprises an apparatus for displaying, for example a video image, a graphic image, numerical information or a combination thereof. The images may be both monochrome and color images. In the latter case the apparatus comprises an image display system having three color channels for, for example the primary colors red, green and blue, each of which accommodates an image display panel.
A device and an apparatus of this type are described in U.S. Pat. No. 5,184,248. The illumination device described in this Patent has a high efficiency, that is to say, a high percentage of the light emitted by the light source is used for illuminating the object which consists of a liquid crystalline image display panel referred to as LCD panel, in which the illumination is also satisfactorily uniform. A bright image having a uniform intensity can be projected on a projection screen by means of an image projection apparatus comprising this device. This is achieved by passing the light beam emitted by the source through an optical integrator comprising a first lens plate having a plurality of first lenses whose height-width ratio corresponds to that of the LCD panel to be illuminated and a second lens plate having a plurality of second lenses which image the light spots formed on the first lenses in a superimposed manner on the LCD panel. It is thereby achieved that the light beam cross-section at the area of the LCD panel exactly fits this panel and that the intensity distribution within this cross-section is uniform. Subsequently the beam is first split in a polarizing system by means of a polarization-sensitive beam splitter into two linearly polarized sub-beams having mutually perpendicular directions of polarization, and subsequently these sub-beams are combined again after one of the sub-beams has undergone a polarization rotation so that its direction of polarization is the same as that of the other sub-beam. Also as regards the polarization, all light from the source can thus be used for illuminating the LCD panel.
In most embodiments of the known illumination device the second lens plate must be imaged in or near the exit plane of the polarizing system, for which image a plurality of lenses is required and, moreover, a given distance between this lens plate and the polarization system is necessary.
U.S. Pat. No. 5,184,248 shows an embodiment in FIG. 31 in which the polarizing system is directly arranged behind the optical integrator and in which no lens system for imaging the second integrator plate is present on the polarizing system, so that this embodiment is shorter and comprises fewer components than the other embodiments. However, the different embodiment supplies a beam having a larger cross-section than that which is supplied by the other embodiments, so that, for example the projection lens system must have a large aperture when this embodiment is used in an image projection apparatus. Moreover, in the devices described in U.S. Pat. No. 5,184,248 the lens plates should have relatively large dimensions and therefore be arranged at a relatively large distance from each other, because the optical integrator is arranged in a beam having a relatively large cross-section.